rebound controls the rate at which your fork will return to its normal height after taking a hit. If you have too little, your fork will feel almost like a pogo stick, where it will "bounce" back at you. If you have too much, your fork will return too slowly, and will "pack up", or stay to low in its travel after successive hits.
Preload is an adjustment for a coil fork. It doesn't change the behavior of a fork, it just changes where it is sitting in its travel. Truly it's only useful for making small geometry changes and for keeping your fork in the correct range of travel (not topping or bottoming out). think of this like adding or removing spacers next to your spring. If you add spacers (add preload) the fork will ride higher. If you remove them, the fork will ride lower.
